To achieve higher energy saving in image forming apparatuses of an electrophotographic system, there is a need for a toner for electrostatic image development (hereinafter may be referred to simply as a “toner”) that is heat-fixable at lower temperature.
Generally, the low-temperature fixability of a toner has a trade-off relation with heat-resistant storage stability, and there is a need for achieving both of them simultaneously. In recent years, as effective technical means for breaking the trade-off relation to improve low-temperature fixability, a method in which a crystalline polyester resin having sharp melting properties is introduced into toner particles is receiving attention.
Particularly, the most ideal form of the introduced crystalline polyester resin to improve its effect is a form in which the crystalline polyester resin in toner particles does not dissolve in a main resin before heat fixation such as during storage of the toner and does dissolve in the main resin during heat fixation. Since the crystalline polyester resin and the main resin dissolve in each other during heat fixation, the main resin is plasticized, so that a very high low-temperature fixation effect is obtained.
However, when a crystalline polyester resin highly compatible with the main resin is introduced into the toner particles, the crystalline polyester resin and the main resin dissolve in each other during production of the toner, so that the obtained toner generally does not have heat-resistant storage stability.
There is a description in Patent Literature 1 that introduction of a crystalline polyester resin into toner particles with the crystalline polyester resin not dissolving in a vinyl resin allows both low-temperature fixability and heat-resistant storage stability to be achieved simultaneously.
However, there is no description about means for dissolving the crystalline polyester resin and the vinyl resin in each other during heat fixation, and therefore it is not sufficient to allow fixation at lower temperature. In addition, there is a problem in that, when most of the crystalline polyester resin is present as an immiscible domain phase in an image after heat fixation, unevenness in gloss occurs in the formed image because of unevenness in size of the domain phase.
However, it is difficult to make a difference between the compatible state of the resins before heat fixation (for example, during storage of the toner) and the compatible state during heat fixation as described above, and there is a need for novel technical means for breaking the trade-off relation.
One possible novel technical means is to add a third component that facilitates dissolution of the crystalline polyester resin into the main resin during heat fixation.
Patent Literature 2 proposes that a compatibilizer having a reactive functional group such as stearyl stearate or glyceryl monostearate is added to a binder resin including a vinyl resin and a crystalline polyester resin.
However, the purpose of this technique is not to make a difference between the compatible state of the resins before heat fixation (for example, during storage of the toner) and the compatible state during heat fixation. Furthermore, addition of the low-molecular weight material likely to be compatible with the crystalline polyester resin may cause the dissolution to gradually proceed before heat fixation (for example, during storage of the toner) along with migration of the low-molecular weight material (molecular migration). Therefore, although short-term heat-resistant storage stability can be ensured, it is difficult to obtain long-term heat-resistant storage stability.
Patent Literature 3 proposes that a hybrid resin of an amorphous polyester resin and a vinyl resin that form a binder resin is added as a compatibilizer to the binder resin.
However, when the amorphous macromolecular material is selected as the compatibilizer, it takes a long time to allow the resins to dissolve in each other because the macromolecular material itself has a certain viscosity in a fixation temperature range, and it is not sufficient to obtain the effects of the compatibilizer in a short time.
Patent Literature 4 discloses a toner in which two types of crystalline polyester resins are used.
However, the aim of this technique is to introduce a highly elastic crystalline polyester resin as a third component to allow this crystalline polyester resin to function as a nucleating agent for the other crystalline polyester resin and is not to facilitate dissolution and plasticization during heat fixation.